Moulding Core Making and Types of Moulding Sands

The moulding is a process of making a cavity (or mould) out of sand by means of a pattern. The molten metal is poured into the moulds to produce castings. Sometimes, a casting is to be made hollow or with cavities in it. Such type of castings requires the use of cores. A core is defined as a sand shape which is exactly similar to the cavities or holes to be provided in the castings. The cores are generally made separately i.e. they are not moulded with the pattern. The process of making cores is called core making.

The moulding materials commonly used in foundry practice are moulding sand, sand binders and sand additives. Quartz and other silica rocks are the source of silica sand which is commonly used for moulding. The binders added to the sand hold the sand grains together, imparts strength, resistance to erosion and breakage and degree of collapsibility. The binders may be clay-type, organic type and inorganic type binders. The sand additives (such as sea coal, wood flour and silica flour) are mixed with heap sand for improving some special features.

The moulding sand must possess the following properties:

1. Porosity or permeability

It is that property of sand which permits the steam and other gases to pass through the sand mould. The porosity of sand depends upon its grain size, grain shape, moisture and clay contents in the moulding sand. If the sand is too fine, its porosity will be low.

2. Plasticity

It is that property of sand due to which it flows to all portions of the moulding box or flask. The sand must have sufficient plasticity to produce a good mould.

3. Adhesiveness

It is that property of sand due to which it adheres or clings to the sides of the moulding box.

4. Cohesiveness

It is that property of sand due to which the sand grains stick together during ramming. It is defined as the strength of the moulding sand.

5. Refractoriness

It is that property of sand which enables it to resist high temperature of the molten metal without breaking down or fusing.

The moulding sands, according to their use, are classified as follows:

1. Green sand

The sand in its natural or moist state is called green sand. It is also called tempered sand. It is a mixture of silica sand with 20 to 30 percent clay, having total amount of water from 6 to 10 percent. The moulds prepared with this sand are called green sand moulds. The green sand moulds are used for small size castings of ferrous and non-ferrous metals.

2. Dry sand

The green sand moulds when baked or dried before pouring the molten metal are called dry sand moulds. The sand in this condition is called dry sand. The dry sand moulds have greater strength, rigidity and thermal stability. These moulds are used for large and heavy castings.

3. Loam sand

A mixture of 50 percent sand grains and 50 percent clay is called loam sand. It is used for loam moulding of large grey-iron castings.

4. Facing sand

A sand used for facing of the mould is called facing sand. It is a specially prepared sand from silica sand and clay, without the addition of used sand.

5. Backing or floor sand

A sand used to back up the facing sand and not used next to the pattern, is called backing sand. The sand which has been repeatedly used, may be employed for this purpose. It is sometimes called black sand because of its black colour.

6. System sand

A sand employed in mechanical sand preparation and handling system is called system sand. This sand has high strength, permeability and refractoriness.

7. Parting sand

A sand employed on the faces of the patterns before moulding is called parting sand. The parting sand consists of dried silica sand, sea sand or burnt sand.

8. Core sand

A sand used for the preparation of cores is called core sand. It is sometimes called oil sand. It is the silica sand mixed with linseed oil or any other oil as binder.